Broom (shrub)
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Brooms are a group of evergreen, semi-evergreen, and deciduous shrubs in the subfamily Faboideae of the pea family Fabaceae, mainly in the three genera Chamaecytisus, Cytisus and Genista, but also in four other small genera (see box, right). Some species of Cytisus are occasionally treated in other genera, Argyrocytisus, Sarothamnus and Spartocytisus. These genera are all closely related and share similar characters of dense, slender green stems and very small leaves, adaptations to dry growing conditions. Most of the species have yellow flowers, but a few have white, orange, red, pink or purple flowers. Two other close relatives are Ulex (Gorse) and Laburnum (Laburnum), but these differ more strongly in appearance from the brooms. Some botanists include Podocytisus caramanica in the genus Laburnum.
All the brooms and their relatives (including Laburnum and Ulex) are natives of Europe, north Africa and southwest Asia, with the greatest diversity in the Mediterranean region. Many brooms (though not all) are fire-climax species, adapted to regular stand-replacing fires which kill the above-ground parts of the plants, but create conditions for regrowth from the roots and also for germination of stored seeds in the soil.
The most widely familiar is the Common Broom (Cytisus scoparius, a.k.a. Sarothamnus scoparius), a native of northwestern Europe, where it is found in sunny sites, usually on dry, sandy soils. Like most brooms, it has apparently leafless stems that in spring and summer are covered in profuse golden-yellow flowers. In late summer, its pea-pod like seed capsules burst open, often with an audible pop, spreading seed from the parent plant. It makes a shrub about 1-3m tall, rarely to 4m. It is also the hardiest broom, tolerating temperatures down to about -25ðC.
The largest species of broom is the Mount Etna Broom (Genista aetnensis), which can make a small tree to 10m tall; by contrast, some other species, e.g. Genista tinctoria, are low sub-shrubs, barely woody at all.
Brooms tolerate and often thrive best in poor growing areas and conditions and need little care; they do though need good drainage and are poor on wet soils.
Common Broom (flower)
They have been widely used as ornamental landscape plants and also for wasteland reclamation (e.g. mine tailings) and sand dune stabilising. Species of broom popular in horticulture are the Purple Broom (Chamaecytisus purpureus; purple flowers), Atlas Broom (Cytisus battandieri, a.k.a. Argyrocytisus battandieri), Dwarf Broom (Cytisus procumbens), Provence Broom (C. purgans) and Spanish Broom (Spartium junceum). Many of the most popular brooms in gardens are hybrids, notably Kew Broom (Cytisus x kewensis, hybrid between C. ardoinii and C. multiflorus) and Warminster Broom (Cytisus x praecox, hybrid between C. purgans and C. multiflorus).
In some areas of North America, the Common Broom, introduced as an ornamental plant, has become naturalised and an invasive weed due to its aggressive seed dispersal; it has proved very difficult to eradicate. Similarly, it is a major problem species in the cooler and wetter areas of southern Australia.
The Plantagenet kings originally used the broom ("planta genista") as an emblem, and took their name from it.
Common Broom (Cytisus scoparius) in flower.